Received 28 January 2005, in revised form 19 September 2005; published online 26 July 2006

Abstract. Transparent motion stimuli allow us to investigate how visual motion is processed in the presence of multiple sources of information. We used stereo random-dot kinematograms to determine how motion processing is affected by the difference in direction and depth of two overlapping motion components. Observers judged whether a noise dot display contained one or two directions of motion. For all disparity differences, performance did not change among angles greater than 60°, but the ability to detect transparent motion fell dramatically as the direction difference decreased below 60°. When a disparity difference was added between the two motion components, detection became easier. We compared these results to an ideal-observer model limited by stimulus uncertainty and low-level sources of internal noise. The resulting measure of efficiency—the ratio of human to model performance—reflects changes in how motion stimuli are being processed. A decrease of both the direction and disparity differences had the effect of decreasing efficiency. These results suggest that the mechanism processing transparent motion may implement a smoothness constraint that tends to combine similar motions into a single percept.

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